Process of making photomat



Patented Nov. 24, 1936 UNITED STATES PROCESS OF MAKING PHOTOMAT Darrel M. Barnes and Hubert D. Dwight, Portland, Oreg.

No Drawing. Application July 8, 1935, Serial No. 30,422

3 Claims. (01. 41-25) This invention relates to a new process of producing a matrix wherein a design is transferred to the material of the matrix by photographically prepared films or plates, thus producing an article which we term a photo-mat which is to be used as a mold in the stereotype casting method of manufacturing metal-alloy printing plates.

It has as one object the simplification of manufacture of the plate; second to considerably reduce production cost; and third to improve the quality of the plate since it omits at least one transfer step heretofore considered necessary and it is well known that each transfer tends to impair the sharpness of design.

In making a stereotype printing plate the usual method is to first produce a photo-engraved zinc etching of the subject matter to be printed and then to place the photo-engraved zinc plate and a matrix card together in a heavy press, where, under intense pressure, the matrix card is forced into the depressions of the zinc plate and is then ready for use as a mold in the stereotype casting box where, when melted lead-alloy is poured upon it, a duplicate of the photo-engraved zinc is formed.

Instead of first manufacturing a photo-engraved zinc printing plate from which to make a matrix, we paint a six, eight, ten or heavier ply uncoated cardboard with plural coats of an animal glue solution containing albumen (white of egg), clear gelatine and gelatinous animal glue. To render this coating material light sensitive we add a light sensitive salt such as ammonium or potassium bichromate dissolved in water which may be rendered alkaline by the addition of aqua ammonia if desired.

Or the coating may be sensitized by the application of the dissolved bichromate salt after the card is coated, instead of incorporating it in the coating material.

When dry the glue coating on the card has the peculiar property of swelling up or expanding (in depth) about one-thirty-second of an inch for a two layer coating when subjected to heat,

such as is present when melted metal is poured on top of it. This swelling is apparently instantaneous; in any event it occurs well before the metal has had an opportunity to set. This fact has a direct bearing on further steps of the process which follow.

We now place our coated card in fiat contact with a translucent photographic positive (or negative) of our subject matter and expose our sensitized card through this translucent element to actinic light which causes the parts open to the light to coagulate and become insoluble in Warm water, the protected parts remaining soluble.

When it is desired to use a graduated tone drawing or natural photograph in translucent form for reproduction, we first expose our light sensitive card through a half tone screen and afterwards expose again through the translucent element of our subject matter.

We then wash out the soluble sections of our exposed coating in luke warm water and allow the card to dry. After coagulation by exposure to actinic light, the remaining insoluble glue on our card will no longer swell up when subjected to heat but only burn in; hence it is necessary to saturate our glue coating with one of the following chemicals dissolved in water to restore its peculiar quality of swelling, and again allow it to dry. The following aqueous solutions have been found suitable:

Bicarbonate of soda 5 to 10% solution Ammonium carbonate 5 to 10% solution Potassium carbonate 5 to 10% solution Sodium carbonate 5 to 10% solution Tartaric acid 5 to 10% solution Citric acid 5 to 10% solution and we believe there are a number of others, perhaps required in more concentrated solutions, such as, strontian earth, calcium earth, lithium earth, magnesium earth, benzoate of soda, that will serve the same purpose more or less perfectly.

We have found it satisfactory to mix the ingredients of the animal glue solution in about the following proportions,--

Parts Albumen (egg) 4 Gelatine 3 Gelatinous animal glue 2 The gelatine and gelatinous animal glue are each dissolved from a grandular form; one ounce avoirdupois weight to eight liquid ounces hot Water; and the egg albumen added when this solution is sufficiently cool.

For sensitization we prefer the ammonium bichromate crystals or the potassium bichromate crystals, a good proportion being 1 ounce dissolved in 4 liquid ounces of water with aqua ammonia added until the solution has turned a lemon yellow. Add one dram of this solution per ounce of our glue solution at about '70 deg. Fahr.

With the card prepared as hereinbefore described or by equivalent method, we are now ready to produce our printing plate, it being understood that the photographic positive may be substituted for by a photographc negative with the expected change in result.

We place our card in the casting box and pour the melted metal on top of it. Immediately that part of the coating which is in relief due to the previously described washing out process of soluble parts, swells up as heretofore described and brings the design out in high relief, particularly sharp and definite, greatly reducing the time and the number of steps necessary to-produce a printing plate by the known method and also producing a plate of high quality.

It may be remarked that gelatines, ordinary liquid glue and albumen have been previously used in conjunction with bichromate salts as a coagulating and coating agent when exposed to actinic light and used as an acid resist on. metals in the Various engraving processes, but we do not know and donot believe that gelatinous animal glue has ever been used in composition therewith. By using the gelatinous animal glue we have attained the described swelling or increase in bulk but without it, it appears to be unattainable in satisfactory measure.

What we claim as new and desire to secure by Letters Patent, 15,--

1. The method of preparing a photo-mat for the direct'casting of a printing plate which consists of preparing a coated card of the character described, photographically transferring a design to the coated card, simultaneously rendering parts of the coated surface insoluble, washing out the soluble portions of the coating to make a matrix of the design and treating the remaining portions of the coating to restore heat sensitiveness impaired by the photographic processes.

2. The method of making a photo mat for direct casting of a printing surface which consists in coating a smooth backing plate with a coating initially possessing the property of swelling under the influence of heat of molten metal, sensitizing the applied coating, photographically transferring a design thereto, Wash- .ing out unexposed portions of the coating to the bare'plate, neutralizing the photographic salts 'in the exposed'portion to restore its expansive quality and thoroughly drying.

3. The method of making a photo mat for direct casting of a printing surface which consists in coating a smooth backing plate with a coating initially possessing the property of swelling under the influence of heat of molten metal, containing bichromatic, light sensitive material, photographically transferring a design thereto, washing out unexposed portions of the coating to the bare plate, neutralizing the sensitive material in the exposed portion to restore the expansive quality of the coating and thoroughly drying.

BARREL M. BARNES. HUBERT D. DWIGHT. 

